1. Field of the Invention
The present invention relates generally to an involute spur gear pair each gear mating with the other and made of a plastic material for use in an environment suffering large temperature changes, a smaller one of the involute gear pair having 16 or less gear teeth. The present invention is applicable more particularly to an involute gear pair each made of a plastic material having a different coefficient of linear expansion.
2. Description of the Related Art
Shifted gears are small gears having 16 or less gear teeth in order to obviate an occurrence of undercutting in their manufacture. The positive shift brings about a shorter dedendum and a larger circular tooth thickness, allowing the occurrence of undercutting to be prevented or suppressed.
FIG. 3 shows a state of engagement of shifted gears having a pressure angle of 20 degrees, consisting of a small gear, i.e., a pinion with 10 gear teeth and a large gear, i.e., a gear wheel with 61 gear teeth. Similarly, FIG. 4 depicts a state of engagement of shifted gears having a pressure angle of 12 degrees.
In the state of the large pressure angle (20 degrees) of FIG. 3, the undercutting of the pinion can be prevented by virtue of the shift. However, this results in a shorter addendum and consequently in a small contact ratio.
On the contrary, in the state of the small pressure angle (12 degrees) of FIG. 4, an addendum becomes longer but undercuts occur in roots of the teeth, resulting in a reduced pinion strength. In the event that such gears are made of plastic, the undercuts become necked, so that resin material, when cooled and contracted upon injection molding, makes die cutting impossible and manufacture of the gears difficult.
In this manner there have not hitherto existed any practical plastic involute gears which are available for use in applications having large temperature changes, with an increased contact ratio, under conditions where the pinion has an extremely small number of gear teeth. Especially, in the case of an involute gear pair, each made of a plastic material having a different coefficient of linear expansion, engagement may often become infeasible due to a change in the environmental temperature.
It is therefore an object of the present invention to develop and provide gears capable of overcoming the above deficiencies.
In order to attain the above object, according to the present invention there is provided an involute gear pair structure including a smaller number of first tooth profiles and a larger number of second tooth profiles, the first tooth profiles meshing with the second tooth profiles for transmission, wherein each of the first tooth profiles has at its addendum an involute tooth profile with the pressure angle xcex11 and has at its dedendum an involute tooth profile with the pressure angle xcex12 where xcex11 less than xcex12, and wherein each of the second tooth profiles has at its addendum an involute tooth profile with the pressure angle xcex13 and has at its dedendum an involute tooth profile with the pressure angle xcex14 where xcex13=xcex12 and xcex14=xcex11.
In the involute gear pair, the number of the first tooth profiles is 16 or less, with the first tooth profiles being shifted in the positive direction, and the number of the second tooth profiles is 25 or more, with the second tooth profiles being shifted in the negative or positive direction or unshifted. The involute gear pair is made of plastic materials.